Minute amounts of contamination, including particles and molecular impurities can adversely affect the microchip fabrication process in the electronics industry. For example, adsorbed molecules such as water and oxygen can lead to undesired native oxide growth on a silicon wafer surface. Other molecular impurities, such as organics and metallics can reduce device performance and limit production yields.
Contaminants may be introduced to the fabrication process through deposition onto the surface of semiconductor wafers or other contamination sensitive devices, such as a glass mask substrates. Deposition may occur during transportation or storage of the wafers between processing steps. Semiconductor device fabrication can be enhanced by minimizing the rate of such deposition. The deposition rate can be reduced by transporting and storing the wafers in mini environment containers.
Semiconductor devices, such as silicon wafers must occasionally be carried or transported between destinations. Such devices are presently transported between clean locations in sealed containers. The protective containers are designed to prevent deposition of undesired particulate material on the clean wafer surfaces. The containers typically enclose an atmosphere of stagnant clean (filtered) air to surround the wafers. The particulate content of the clean air is minimized by installing the wafers in the container in a clean air environment. However, such stagnant clean air has been found to deposit small amounts of particulate matter on the wafers during transport. Also, the air contains large quantities of uncontrolled molecular impurities, such as ambient organic molecules, oxygen and water which may contaminate the wafers.
The most commonly used containers for semiconductor wafers consist of simple plastic boxes to enclose "boats" of wafers. The design of such containers may in some cases conform to SEMI Standard Mechanical InterFace (SMIF) requirements. However, such containers have been found to permit unacceptably high rates of contaminant deposition over time. Many contaminants may become sealed in the containers with the wafers. Also, contaminants may become released from the internal walls of the container over time. In most cases no internal gas purge or pressurization is provided to these containers. Therefore, additional contaminants may slowly enter the container through imperfect seals.
A recent improvement in wafer transporting uses nitrogen purging to create a higher purity, low particle level mini environment. One such container is marketed by Portable Clean Rooms. The container was advertised in the January 1994 issue of MICROCONTAMINATION and has been featured in Solid State Technology (November 1993) and CryoGas International (March 1994). This device uses pressurized gaseous nitrogen contained in an attached mini cylinder to provide a continuous filtered purge to the container. The wafer container is constructed primarily from plastic. A related patent (U.S. Pat. No. 4,668,484) has been filed by the manufacturer. A compressed gas cylinder is mounted above the wafer container. The gas cylinder is not intended to be re-used. It must be discarded when empty. A replacement cylinder must then be purchased. The system is intended to be used for semiconductor wafer storage and for carrying wafers between clean locations. The container is designed to contain 50 to 200 mm diameter silicon wafers. The container includes a pressure switch and an LED indicator connected to the wafer container. The indicator blinks when the switch senses positive pressure in the container. The system is 54 cm tall, with a 17 cm.times.24 cm footprint. The (unloaded) weight of the system is 4,740 gm (10 pounds). Useful purge life, flowrates and nitrogen storage capacity are given in a specification release, "The Portable Clean Room.TM. Wafer Transport System" by Portable Clean Rooms.
A similar purged container for silicon wafers was described by Yabune, et al., "Isolation Performance of a Wafer Transportation System Having a Continuous N.sub.2 Gas Purge Function", Proceedings, Institute of Environmental Sciences, 1994, pp 419-424. The Yabune, et al., container also uses an attached mini cylinder of pressurized nitrogen to purge the wafer container. The Yabune, et al., system uses an aluminum container and a high purity all-metal gas distribution system. Yabune, et al., have demonstrated a reduction in native oxide growth rate and an improved device performance when the purged storage system is used.
Asyst Technologies, Inc. markets SMIF pods for silicon wafers and other semiconductor devices. The Asyst device does not provide for continuous purging of the pods. However, the Asyst device provides an optional pod sealing system which encloses pressurized nitrogen inside the pod. The positive pressure is intended to minimize exposure of the wafers to external molecular and particulate contaminants. However, the pure environment cannot be maintained indefinitely. Imperfect seals cause the internal pressure of the pod to decay over a period of time. See SMIF-Pods, Asyst Technologies, Document #2100-1015-01.
U.S. Pat. No. 5,351,415 discloses a container for storage or transport of semiconductor wafers that uses ionized gas, such as gaseous nitrogen. The nitrogen is supplied from a cylinder of compressed gas that is typical in the industry. The compressed gas cylinder is not affixed to the container, but is connected through a gas line.
The prior art has attempted to provide a solution to the problem of storing and transporting contamination-sensitive components, such as semiconductor wafers, in a human operator transportable container. However, the prior attempts suffer from limited capacity of inerting gas available for such containers, the limitation on the purity of the inerting gas particularly on a steady state basis during use of the capacity of inert gas available, the inability to vary the rate of inert gas flow, and the lack of refill capability. The present invention, as described below, overcomes all of these disadvantages of the prior art as will be described in greater detail.